This invention relates to switching circuits. More particularly, this invention relates to switching circuits having independent source and sink components.
One conventional method of switching ON an output circuit, e.g., the output transistor of a switching regulator, is to drive its base high with a fixed current source coupled to Vcc. After the circuit is switched ON, an anti-saturation circuit may be used to detect the voltage across the output circuit, and, when necessary, to shunt excess drive to ground or into the collector of the output circuit. When the output circuit is switched OFF, a fixed current sink may be used to shut OFF the output circuit by sinking current from the base of the output circuit to ground.
This approach, however, has several potential problems. One problem is that the source and sink currents required to drive the output circuit are often excessive under lighter loads. Under such conditions, power is wasted when the output circuit is switched ON by a source circuit to a greater extent than necessary, and when the output circuit is switched OFF by a sink circuit to a greater extent than necessary. In addition, the extra source current supplied by the source circuit under lighter loads may slow the switch OFF time of the source circuit. Because the output circuit will not switch OFF as long as the source circuit is still ON, a delay in switching OFF the source circuit lengthens the switch-OFF period of the output circuit.
Another potential problem with conventional switching circuits is that the sink circuit often must be switched OFF itself before the output circuit can be switched back ON. This also may add extra time delay to the switch-ON period of the output circuit.
One other problem that may occur is that additional circuits may be required to prevent the output circuit from saturating. These circuits, which typically shunt extra drive current to ground, rely on an assumed saturation voltage for reference. Thus, these circuits typically have no compensation for variations in transistor characteristic or load current.
A problem may also exist if the emitter of the output circuit is held above ground during the switch OFF period, and the sink circuit grounds the base of the output circuit. This arrangement can backward bias the emitter-base junction of the output circuit and cause an undesirable leakage path from the emitter of the output circuit through its base to ground via the sink circuit. This leakage can damage the output circuit.
Finally, if the load is short-circuited, the source circuit may be forced to provide maximum drive current. This can cause the output circuit to operate at excessively high currents, potentially causing damage to the output circuit.